Thank you all for the replies. In my case, I think I found the culprit. I'm using Amazfish to sync with my smartwatch. The app requires libicu 52. Since libicu was updated during the 3.0.3 upgrade to version 63, I have additionally installed the old library (provided by piggz). However, this seems to be the issue. While Amazfish is running, the battery drain is massively increased (like around 9 hours idle time with a full battery instead of the usual 30), as soon as I close the app it is greatly increasing again.